1. Field of the Invention
This invention relates to an oxygen sensor element, and more particularly to an oxygen sensor element which detects oxygen concentration by using a concentration cell having electrodes carried by separate solid electrolytes.
2. Description of the Prior Art
Heretofore, for example, to detect oxygen concentration in automobile exhaust gas, various oxygen sensors have been used; namely, oxygen sensors having electrodes disposed on opposite surfaces of a cylindrical solid electrolyte, oxygen sensors having a built-in heater for ensuring regular operation even when exposed to low-temperature exhaust gas whose temperature is as low as below 300.degree. C., or lean burn type oxygen sensors suitable for gas with an air-fuel ratio indicating excess air. As to the shape of the solid electrolyte in oxygen sensors, a cylindrical solid electrolyte with a bottom board and a laminated structure formed from plate-like or film-like solid electrolytes overlaid one above the other has been proposed and used.
The conventional arrangement of disposing a pair of electrodes on opposite surfaces of a film-like solid electrolyte has a shortcoming in that the difference in both the thermal expansion and the firing shrinkage between the solid electrolyte and the electrodes tend to cause stress in the solid electrolyte. When a sudden temperature change occurs or after continuous operation over a long period of time, various adverse effects result, such as the occurrence of cracks in the solid electrolyte, an increased error in the output electromotive force, or fracture of the oxygen sensor element. Such shortcomings are particularly noticeable in the case of thin film-like solid electrolytes.
FIG. 1 shows the structure of a laminated type oxygen sensor element according to the prior art, wherein film-like or plate-like solid electrolytes are overlaid one above the other, and a reference electrode 4 is spaced from a heater 11 by a gap portion 7 for introducing a reference gas such as air into the sensor element. Accordingly, the efficiency of heat transmission from the heater 11 to the reference electrode 4 is low, and the reduction of impedance between a solid electrolyte 2 and the reference electrode 4 is delayed, especially immediately after the start of the sensor operation. Thus, the structure of FIG. 1 has a shortcoming of being slow when beginning the sensing operation.